### Please describe the problem.

I have not checked if it is legit to have an "empty" port number in the (http) URL but I see that `git` itself handles it fine, but git-annex is not happy:

```
❯ git clone https://datasets.datalad.org:/dbic/QA/.git/
Cloning into 'QA'...
warning: redirecting to https://datasets.datalad.org/dbic/QA/.git/
remote: Enumerating objects: 61661, done.
remote: Counting objects: 100% (61661/61661), done.
remote: Compressing objects: 100% (23181/23181), done.
remote: Total 61661 (delta 31300), reused 56651 (delta 26299)
Receiving objects: 100% (61661/61661), 33.27 MiB | 25.03 MiB/s, done.
Resolving deltas: 100% (31300/31300), done.

❯ git annex get sub-emmet/ses-20180508/anat/sub-emmet_ses-20180508_acq-MPRAGE_T1w.nii.gz

  Remote origin not usable by git-annex; setting annex-ignore

  https://datasets.datalad.org:/dbic/QA/.git//config download failed: Unsupported url scheme https://datasets.datalad.org:/dbic/QA/.git//config
get sub-emmet/ses-20180508/anat/sub-emmet_ses-20180508_acq-MPRAGE_T1w.nii.gz (from datasets.datalad.org...) 
(scanning for annexed files...)   
ok
(recording state in git...)
```

so it got the file only after enabling type=git special remote for the same location but with correct URL.

I think it would be nice if git-annex was as robust as git in such cases to avoid "late surprise".

Backstory: Happened to a user trying to access some NWB files on gin for DANDI project, here I used different/simpler/faster URL

[[!meta author=yoh]]
[[!tag projects/dandi]]

> [[fixed|done]] --[[Joey]]
